An important feature of petroleum refining is the conversion of high molecular weight petroleum feeds such as vacuum gas oil (VGO) and heavier molecules to more useful and valuable products of lower molecular weight and boiling point. The fluid catalytic cracking unit (FCCU) is one of the most important units in the refinery, able to produce a large proportion of the gasoline pool as well as producing large volume of distillate range cracked products useful for the manufacture of diesel fuel. In the FCCU, high molecular weight feeds are contacted with fluidized catalyst particles with the conditions of the cracking process controlled according to the type of product desired; conditions such as temperature and contact time are controlled to maximize the products desired and minimize the formation of less desirable products such as light gases and coke.
Miscellaneous FCC riser and reactor designs have been utilized but with the advance of zeolitic cracking catalysts with greatly improved cracking activity, most modern FCC reactors utilize a short-contact time cracking configuration in which the amount of time that the catalyst and the FCC feedstream are in contact is limited in order to minimize the excessive cracking which results in the increased production of less valued products such as light hydrocarbon gases as well as increased coking deposition on the cracking catalysts. Current designs carry out the cracking of the feed in a riser which is a substantially vertical pipe with a feed injection zone at the bottom into which hot catalyst from the regenerator is fed to meet the incoming feed which is injected into the mix zone through nozzles with aid of steam. The regenerated catalyst enters the riser below the feed mix zone and is lifted up into the mix zone with lift gas. In the riser the vaporized feed is cracked into smaller molecules by contact and mixing with the hot catalyst; the cracking reactions take place in the catalyst riser within 10 seconds, typically 2-4 seconds. The mixture of hydrocarbon vapors and catalyst flows upward to enter the reactor vessel which now functions as a disengager to permit separation of the spent catalyst from the cracked hydrocarbon vapors. After disengagement from the catalyst, the spent catalyst flows downward through a steam stripping section to remove any hydrocarbon vapors before the spent catalyst returns to the catalyst regenerator. In the regenerator the coke which accumulates on the catalyst particles as a result of the carbon rejection which is the characteristic feature of the process is burned off with air to restore catalyst activity and selectivity as well as providing heat by the exothermic combustion of the coke to maintain a heat balance in the unit with the endothermic cracking reactions.
Most FCC reactor designs use mechanical cyclones internal to the reactor to disengage or separate the catalyst from the hydrocarbon reactor products as quickly and efficiently as possible. This rapid separation process has the benefits of both minimizing post-riser reactions between the catalyst and the hydrocarbons as well as providing a physical means for separating the products to be sent for further processing from the spent catalyst which is sent to the regenerator prior to being reintroduced into the riser as regenerated catalyst to continue the FCC cycle.
After separation from the spent catalyst, the FCC reactor stripping section utilizes a stripping medium, usually steam, to strip hydrocarbons from the spent FCC catalyst prior to the catalyst being sent to the regenerator. In the FCC regenerator, the spent catalyst is typically subjected to temperatures from about 1100 to about 1400° F. (about 590 to 760° C.) in order to regenerate the catalyst activity. The hydrocarbons that are not effectively stripped off of the catalyst in the stripper pass to the regenerator resulting in an increased combustion load on the FCC regenerator as well as having several other adverse impacts to an FCC unit: insufficient stripping of hydrocarbons in the FCC stripper can be a direct cause of loss in product output as well as resulting in increased regenerator emissions and other detrimental effects. Additional combustion of non-coke cracking products in the regenerator is undesirable as it increases contaminant concentrations in the regenerator flue gas and/or increases the regenerator flue gas rate resulting in increased air pollutant emissions from the FCC unit. Additionally, an inefficiently designed FCC stripping section will result in the use of an excess amount of steam in the FCC stripper and reactor. This excess steam can result in a decrease overall hydrocarbon processing capacity in the associated FCC fractionator tower as well as increasing the amount of water that must be removed from the hydrocarbon product and subsequently treated prior to disposal or reuse.
There have been apparatus designs intended to improve the catalyst/stripping gas contact in the FCC stripper. Many “disc and donut” stripper tray designs have been proposed to improve the stripping process associated with the “annular riser” FCC reactors in which the riser entering at the bottom of the reactor and rises up through its central axis. Examples of annular tray designs can be seen in U.S. Pat. No. 7,744,746 (Cunningham), U.S. Pat. No. 5,531,884 (Johnson), and U.S. Pat. No. 6,248,298 (Senior). Other stripper designs may be used in units with external risers. Stripper packings have also been proposed to increase the stripping efficiency of the stripper, for example, as in U.S. Pat. No. 5,716,585 (Senegas).
Hot stripping of the spent catalyst has been recognized as carrying advantages. U.S. Pat. No. 4,789,458 (Haddad), for example, describes an FCC process which includes high temperature stripping (hot stripper) to control the carbon level, hydrogen level, and sulfur level on the spent catalyst, followed by single or multi-stage regeneration.
U.S. Pat. No. 6,139,720, (Lomas) maximizes the production of carbon monoxide as a combustion off-gas by the use of a hot stripping zone arrangement that lifts hot stripped catalyst to the top of a bubbling-bed regeneration zone by an oxygen-starved lift stream. The process delivers spent catalyst with about 1 wt % of coke to the reaction zone and a spent combustion gas or flue gas stream having a CO2 to CO ratio of at least 1.
For decades FCC has been the major air pollution contributor in refining. The heavy oil (high boiling range) feeds to the FCCU typically contain molecules containing high proportions of carbon (rejected as coke during the cracking) as well as of sulfur and nitrogen compounds which convert under the reducing conditions of the cracking section to H2S, NH3 and HCN which leave the reactor with the cracked products. These contaminants have typically been handled in a relatively environmentally and economically friendly way by water washing, amine absorption, sour water stripping and sulfur plant processing steps. The main environmental and economic issue however has arisen from the operation of the regenerator. The flue gas from the regenerator can result in very large NOx, SOX, CO and particulate emissions which need to be controlled and reduced to comply with environmental regulations as well as an aspect of prudent and considerate refinery management. The refining industry has been spending major amounts money and effort in managing these emissions by means such as partial recovery/conversion in CO Boilers, Selective Catalytic Reduction (SCR) and Wet Gas Scrubbing to remove gaseous contaminants and with tertiary cyclones, electrostatic precipitators and baghouses to reduce particulate emissions. These environmental control measures have in themselves been expensive to implement in addition to the basis cost of the regenerator which, as a major component of the cracker is itself expensive. For a typical present day refinery the FCC regenerator and its flue gas recovery section can easily cost more than US$1B to construct and this while it generates negative value by consuming chemicals and creating waste.
Improvements in the operation of the regenerator with a view to reducing emissions by the use of oxygen enrichment have been proposed. U.S. Pat. No. 5,908,804 for examples, discloses the use of a regenerator operating with an oxygen-inert gas mixture in which the oxygen concentration is at least about 24 vol. % at a temperature at which most of the coke burns to a mixture of carbon monoxide and carbon dioxide, so substantially oxidizing the reduced nitrogen species that come from the coke to nitrogen oxides which are then passed through a reducing zone to convert them to elemental nitrogen.
An alternative proposal put forward in EP 1 201 729 (Menon) has been to inhibit the formation of nitrogen oxides in a carbon monoxide boiler by using fuel gas to burn carbon monoxide downstream of the regenerator by introducing an oxygen-enriched gas into the boiler, preferably with a nitrogen-enriched gas simultaneously added to the boiler.
The Praxair company has taken an interest in oxygen enrichment in the FCC regenerator: U.S. Pat. No. 7,470,412 (Rosen) discloses a process in which a hot oxygen stream is fed into a catalyst regenerator flue gas stream to remove carbon monoxide from the stream. NOx precursors such as NH3 and HCN are converted into N2 and if NOx is present in the flue gas stream the addition of the hot oxygen stream lowers the amount of NOx present. Praxair also advertises its oxygen enrichment technology in a company brochure, “Praxair+FCC Oxygen Enrichment” (available online).
These earlier proposals have, however, been along rather conventional lines in tacitly accepting the inevitability of the regenerator, either in the form of a dense bed regenerator as typically found in Kellogg Orthoflow™ units or a riser type regenerator as in UOP units.